Laboratory Investigations in Microbiology

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Chapter 29: Food production: Yogurt

Perhaps one of the oldest known uses for microorganisms is in the production of foods. Microorganisms play an important role in the production of most dairy products (cheese, yogurt, sour cream) and alcoholic beverages as well as vegetables such as sauerkraut. In most instances, the microbial process that is sought after is fermentation. Because of the various types of fermentations that microbes can do. many different flavors and products can be made. In principle, most food production starts with the fermentation of a sugar source. This is usually lactose in dairy products and sucrose or fructose in alcoholic beverages. Depending on the microbe used, the end product of fermentation may be alcohol (yeast), lactic acid (most milk bacteria), or other flavoring agents. 

Dairy products are produced using a starter culture of lactose-fermenting Lactobacillus and Streptococcus. These microbes produce lactic acid as their sole fermentation end product, acidifying the milk. The acids cause the milk protein (casein) to curd (clump). The length of fermentation and the strength of the acids produced make for different degrees of curding. Initially, milk will thicken, as in yogurt. Eventually, the curd precipitates out (or can be squeezed) to extract the liquid portion of milk, called the whey. In cheese production, the whey is discarded and the curds are pressed and allowed to age. Aging may involve inoculation with further microbes that can produce a buttery flavor (butyric acid fermentation), more acids (hard cheeses), or proteolytic enzymes that break down casein, making for a softer cheese such as Brie. 

In each case, the production depends on having the proper starter culture of bacteria. In this experiment, we will use a small amount of Dannon Yogurt to provide the starter culture. The bacteria used for yogurt production are Lactobacillus bulgaricus and Streptococcus thermophilus. Milk is mixed with some powdered skim milk (additional sugar and protein) and heated to near boiling to kill potential pathogens. It is then cooled to 45C, inoculated with starter culture, and incubated for 4 - 6 hours at 45C. This temperature also prevents growth of most pathogens. 

Materials per lab group
  1. Measure out 100 ml of heated milk (80 C). Add nonfat dry milk powder
  2. Heat to near-boiling (80C) for 5 minutes, stirring constantly using a tongue depressor
  3. Place your beaker in the 45C water bath for 5 minutes.
  4. Add 0.5 ml yogurt (cut off the tip of a 1 ml pipette tip to form a larger opening) to the warm milk. Stir well. 
  5. Pour mixture into the yogurt container. Put the lid on and place container into 45C water bath.
  6. Check your yogurt every 2 hours. When it has firmed up, place your jar into the lab refrigerator. (Note: The last time you can come in to check this is at 5:30 pm; Lab C will not be able to do this).


  1. Check the pH of a small sample of your yogurt with a pH meter or other pH indicator.
  2. Smell your yogurt. If it smells bad, throw it out. :(
  3. If it smells OK, taste your yogurt. If desired, bring some jelly to flavor your yogurt. 

Printable Data Sheet & Review Questions

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